Boil 1/4 cup of water in a small pan.
Mix a piece of unflavored/clear gelatin into the water, then use a Popsicle stick to stir the mixture. Add a few of drops of vegetable oil to the mix to help keep it soft when warm. This also delays the mixture's solidifying process. Once well blended, allow the mixture to slightly cool down. Expect the mixture to thicken.
Test the gelatin mixture if it's warm enough for skin application using the Popsicle stick. Touch the slightly cooled down mixture with your finger to see if it's already safe to apply to the skin. Don't allow the mixture to entirely cool down as this will readily solidify the mixture before you can apply it to the skin.
Prepare the actor's skin for the attachment of the initial layer of the fake scar. Dab alcohol on the part of the skin where the scar will be attached or clean the skin with soap and water. This process removes excess skin oil prior to the attachment of the fake scar.
Apply a glob of the warm gelatin mixture onto the skin using the Popsicle stick or a barbecue stick for smaller or thinner scars. Allow the scar-shaped glob to solidify on the skin. Avoid too much movement on the part of the body where the scar was placed until it completely cools down and solidifies.
Apply a second coating of another glob of the warm gelatin mixture. Finalize the scar's shape before the mixture cools down and solidifies on the skin.
Apply a brush-on, powder-based foundation makeup or a liquid foundation makeup over the fake scar to give it the appearance of human skin. You may use a makeup brush or sponge or an eyeshadow applicator. Make sure you use the right foundation color based on the actor's skin tone.